In the Japanese Industrial Standards (JIS), Oil-tempered Wire for Valve Springs (SWO-V: JIS G3561), Chromium-Vanadium Steel Oil-tempered Wire for Valve Springs (SWOCV-V: JIS G3565), and Silicon-Chromium Steel Oil-tempered Wire for Valve Springs (SWOSC-V: JIS G3566), etc. are specified as wires for valve springs of internal combustion engines. Among them, SWOSC-V has been mainly used because it has a higher fatigue resistance and higher sag resistance.
From the viewpoint of environmental protection and resource saving, cleanliness of the exhaust gas and fuel efficiency are always required on automobiles. Weight reduction of an automobile greatly contributes to those requirements, and thus weight reduction of automotive parts is always pursued.
Several new wires for valve springs have been proposed so far having high fatigue resistance and high sag resistance. In the Unexamined Publication No. H8-176730 of Japanese Patent Application, an oil-tempered wire for high-strength valve springs is proposed in its claim 1 as: using a steel including 0.5-0.8 wt % C, 1.2-2.5 wt % Si, 0.4-0.8 wt % Mn, 0.7-1.0 wt % Cr and inevitable impurities with iron balance, where Al content is no more than 0.005 wt % and Ti content is no more than 0.005 wt % in the inevitable impurities (claim 1). The steel is heated to a temperature in the range 950-1100.degree. C., then quenched and tempered. In the Publication, oil-tempered wire including 0.05-0.15 wt % V (claim 2), and oil-tempered wire including, in addition to that, one or more among 0.05-0.5 wt % Mo, 0.05-0.15 wt % W and 0.05-0.15 wt % Nb (claim 3) are also proposed. In the Unexamined Publication No. H971843 of Japanese Patent Application by the same applicant, a high toughness oil-tempered wire for valve springs is proposed as using the same steel as above and the residual austenite (.gamma.) after quenching and tempering is suppressed to 1-5 vol % (claims 1 and 2). In the Publication, another oil-tempered wire is proposed whose microscopic structure is required to be so that the density of carbide particles no smaller than 0.05 .mu.m in its diameter is no more than five pieces/ .mu.m.sup.2 in the microscopic photograph (claims 3 and 4). Combination of these are proposed in claims 5 and 6. The manufacturing method is disclosed as follows. In case of claims 1, 2, 5 and 6, the heating speed in tempering is no smaller than 150.degree. C./sec and the maximum heating temperature in tempering is within 450-600.degree. C. The time period from the beginning of heating to the beginning of cooling by water or the like is no longer than 15 sec. In case of claims 3, 4, 5 and 6, the heating speed in hardening is no smaller than 150.degree. C./sec, and the maximum heating temperature in hardening is no higher than 1100.degree. C. but no lower than the temperature T(.degree. C.) calculated as T=500+750.multidot.C (Carbon) +500.multidot.V (Vanadium). The time period from the beginning of heating to the beginning of cooling by water or oil is no longer than 15 sec.
Most of the measures proposed so far are about the material steels or, at most, about wires (oil-tempered wires), but no measure has been taken to the stage of manufacturing the final product, that is the valve spring, in order to endow high fatigue strength and high sag resistance. If the spring manufacturing process is inadequate, even the best material cannot make a good valve spring, and such an inadequate process may make it difficult to manufacture an appropriate valve spring, and may further, in some cases, deteriorate its fatigue strength or sag resistance.
The present invention addresses such problems, and one of the objects is to provide a valve spring having an improved fatigue strength from conventional ones by choosing the best material and then by applying an appropriate manufacturing process suitable to the material chosen. In concrete, the present invention provides a valve spring having the durability of more than 5.times.10.sup.7 times under the repeated stress of .tau.=60 .+-.51 kgf/mm.sup.2 in its maximum shear stress at the wire. In the present invention, the delayed fracture is also taken into consideration and its durability is addressed.
Another object of the present invention is to provide a valve spring having a lesser sag while possessing no less fatigue strength by choosing the best material and then by applying an appropriate manufacturing process suitable to the material chosen. In concrete, the present invention provides a valve spring yielding the residual shear strain .gamma. of less than 5.times.10.sup.-4 when left 48 hours at the temperature of 120.degree. C. with the maximum shear stress of .tau.=90 kgf/mm.sup.2 in the wire surface. In the present invention, the delayed fracture is also taken into consideration and its durability is addressed.